Coronary artery disease-(CAD) is the major cause of death in Type 2 diabetic and metabolic syndrome patients (i.e. patients that fall within the ‘deadly quartet’ category of impaired glucose tolerance, insulin resistance, hypertriglyceridemia and/or obesity).
The hypolipidaemic fibrates and antidiabetic thiazolidinediones separately display moderately effective triglyceride-lowering activities although they are neither potent nor efficacious enough to be a single therapy of choice for the dyslipidaemia often observed in Type 2 diabetic or metabolic syndrome patients. The thiazolidinediones also potently lower circulating glucose levels of Type 2 diabetic animal models and humans. Studies on the molecular actions of these compounds indicate that thiazolidinediones and fibrates exert their action by activating distinct transcription factors of toe peroxisome proliferator activated receptor (PPAR) family, resulting in increased and decreased expression of specific enzymes and apolipoproteins respectively, both key-layers in regulation of plasma triglyceride content. Fibrates, on the one hand, are PPARα activators, acting primarily in the liver. Thiazolidinediones, on the other hand, are high affinity ligands for PPARγ acting primarily on adipose tissue.
Adipose tissue plays a central role in lipid homeostasis and the maintenance of energy balance in vertebrates. Adipocytes store energy in the form of triglycerides during periods of nutritional affluence and release it in the form of free fatty acids at times of nutritional deprivation. The development of white adipose tissue is the result of a continuous differentiation process throughout life. Much evidence points to the central role of PPARγ activation in initiating and regulating this cell differentiation. Several highly specialised proteins are induced during adipocyte differentiation, most of them being involved in lipid storage and metabolism. The exact link from activation of PPARγ to changes in glucose metabolism, most notably a decrease in insulin resistance in muscle, has not yet been clarified. A possible link is via free fatty acids such that activation of PPARγ induces Lipoprotein Lipase (LPL), Fatty Acid Transport Protein (FATP) and Acyl-CoA Synthetase (ACS) in adipose tissue but not in muscle tissue. This, in turn, reduces the concentration of free fatty acids in plasma dramatically, and due to substrate competition at the cellular level, skeletal muscle and other tissues with high metabolic rates eventually switch from fatty acid oxidation to glucose oxidation with decreased insulin resistance as a consequence.
PPARα is involved in stimulating β-oxidation of fatty acids. In rodents, a PPARα-mediated change in the expression of genes involved in fatty acid metabolism lies at the basis of the phenomenon of peroxisome proliferation, a pleiotropic cellular response, mainly limited to liver and kidney and which can lead to hepatocarcinogenesis in rodents. The phenomenon of peroxisome proliferation is not seen in man. In addition to its role in peroxisome proliferation in rodents, PPARα is also involved in the control of HDL cholesterol levels in rodents and humans. This effect is, at least partially, based on a PPARα-mediated transcriptional regulation of the major HDL apolipoproteins, apo A-I and apo A-II. The hypotriglyceridemic action of fibrates and fatty acids also involves PPARα and can be summarised as follows: (I) an increased lipolysis and clearance of remnant particles, due to changes in lipoprotein lipase and apo C-III levels, (II) a stimulation of cellular fatty acid uptake and their subsequent conversion to acyl-CoA derivatives by the induction of fatty acid binding protein and acyl-CoA synthase, (III) an induction of fatty acid β-oxidation pathways, (IV) a reduction in fatty acid and triglyceride synthesis, and finally (V) a decrease in VLDL production. Hence, both enhanced catabolism of triglyceride-rich particles as well as reduced secretion of VLDL particles constitutes mechanisms that contribute to the hypolipidemic effect of fibrates.
PPARδ activation was initially reported not to be involved in modulation of glucose or triglyceride levels. (Berger et al., j. Biol. Chem., 1999, Vol 274, pp. 6718-6725). Later it has been shown that PPARδ activation leads to increased levels of HDL cholesterol in db/db mice (Leibowitz et al. FEBS letters 2000, 473, 333-336). Further, a PPARδ agonist when dosed to insulin-resistant middle-aged obese rhesus monkeys caused a dramatic dose-dependent rise in serum HDL cholesterol while lowering the levels of small dense LDL, fasting triglycerides and fasting insulin (Oliver et al. PNAS 2001, 98, 5306-5311). The same paper also showed that PPARδ activation increased the reverse cholesterol transporter ATP-binding cassette A1 and induced apolipoprotein A1-specific cholesterol efflux. The involvement of PPARδ in fatty acid oxidation in muscles was further substantiated in PPARα knockout mice. Muoio et al. (J. Biol. Chem. 2002, 277, 26089-26097) showed that the high levels of PPARδ in skeletal muscle can compensate for deficiency in PPARα.
Recently, two different transgenic mouse models over-expressing PPARδ in either adipose tissue (Cell 2003, 113, 159-170) or in muscle tissue (FASEB J. 2003, 17, 209-226) have both shown up-regulation of genes (LPL, FABP, FAT, CD36, CPT1b, and ACS) and proteins (UCP-2) responsible for lipid uptake and metabolism and energy uncoupling. Both types of mice had reduced adipose tissue and were protected against high fat diet induced body weight gain. Further, pharmacological treatment of both high fat diet induced Insulin resistant mice and diabetic ob/ob with the potent PPARδ agonist GW501516 showed lowering of plasma glucose and insulin and improved insulin sensitivity (PNAS 2003, 100, 15924-15929). In vivo increased oxygen consumption suggesting fuel-switch from glucose to FFA, as well as FFA oxidation In skeletal muscle was demonstrated both in vivo and in vitro. Supportive for the hypothesis of skeletal muscle being the major target organ were two publications on in vitro treatment of C2C12 muscle cells with GW501516 showing regulation of genes involved with TG hydrolysis and FFA oxidation (LPL†, ACS4†, CTP1†), preferential lipid utilization (PD K4†), energy expenditure (UCP1\, -2\, -3\) and lipid efflux (ABCA1/G1\) (BioChem. Biophys. Acta 2003, 1633, 43-50; Mol. Endocrin. 2003, 17, 2477-2493). Direct and an indirect mechanisms recently demonstrated prompted the authors to suggest that “PPARδ and its ligands may serve as therapeutic targets to attenuate inflammation and slow the progression of atherosclerosis” (Science 2003, 302, 453-457).
Taken together these observations suggest that PPARδ activation is useful in the treatment and prevention of cardiovascular diseases and conditions including atherosclerosis, hypertriglyceridemia, and mixed dyslipidaemia as well as type 2 diabetes.
A number of PPARδ compounds have been reported to be useful in the treatment of hyperglycemia, hyperlipidemia and hypercholesterolaemia (WO 01/00603, WO 02/59098, WO 03/084916, WO 03/074050, WO 03/074051, WO 03/074052, WO 03/035603, WO 03/97607, WO 04/005253, WO 03/33493, WO 03/16291, WO 02/76957, 02/46154, WO 03/16265, WO 02/100812, WO 02/98840, WO 02/80899, WO 02/79162, WO03/072100, WO 01/25181, WO 02/14291, WO 01/79197, WO 99/4815, WO 97/28149, WO 98/27974, WO 97/28115, WO 97/27857, WO 97/28137, WO 97/27847).
Glucose lowering as a single approach does not overcome the macrovascular complications associated with Type 2 diabetes and metabolic syndrome. Novel treatments of Type 2 diabetes and metabolic syndrome must therefore aim at lowering both the overt hypertriglyceridaemia associated with these syndromes as well as alleviation of hyperglycaemia.
This indicate that research for compounds displaying various degree of PPARα, PPARγ and PPARδ activation should lead to the discovery of efficacious triglyceride and/or cholesterol and/or glucose lowering drugs that have great potential in the treatment of diseases such as type 2 diabetes, dyslipidemia, syndrome X (including the metabolic syndrome, i.e. impaired glucose tolerance, insulin resistance, hypertrigyceridaemia and/or obesity), cardiovascular diseases (including atherosclerosis) and hypercholesteremia.